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Old 01-01-2010, 12:31 AM   #1
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Default Head Protein life span

I've heard Jamil say from some of the BN podcasts that head forming proteins only form once, then they are destroyed. Does anyone know the science behind this? Basically I'm questioning whether its BS or if someone has actually looked at the lifespan of the head forming proteins.

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Old 01-01-2010, 12:46 AM   #2
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This thread is worth a read. Essentially, I think it's BS. They get reabsorbed into solution... at least post-fermentation.
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Old 01-01-2010, 12:57 AM   #3
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Excellent thread. Thanks for the reference and the experiment.

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Old 01-01-2010, 01:44 AM   #4
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Originally Posted by blue800 View Post
I've heard Jamil say from some of the BN podcasts that head forming proteins only form once, then they are destroyed. Does anyone know the science behind this?
Polypeptides, proteins, and iso-alpha-acids all combine to form a good beer head. This should help explain the importance of protein in beer foam. From Beer: quality, safety and nutritional aspects By E. Denise Baxter, Paul S. Hughes:

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Foaming during beer production can lead to the irreversible loss of foam-active species. Irreversible denaturation of proteins, a process perhaps essential to the formation of beer foam, generally reduces protein solubility. Thus foaming will result in a loss of a proportion of foam-active proteins from the bulk beer. Paradoxically, the use of silicone antifoam agents during fermentation is beneficial to final beer foam stability....This is on account of the suppression of foam head during fermentation, with attendant reduced losses of protein.
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This thread is worth a read. Essentially, I think it's BS. They get reabsorbed into solution... at least post-fermentation.
You're only partially correct. Some of the essential foam forming agents (e.g., polypeptides and iso-alpha-acids) are reabsorbed post-fermentation (i.e., not lost to krausen deposits) but the foam positive proteins do NOT get reabsorbed since they are, well, denatured. A subtle yet important distinction.

So...Yes, as much as I dislike his approach to brewing science, Jamil is correct in saying that an essential foam positive agent (protein) is inherently destroyed upon foam creation.
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Old 01-01-2010, 01:40 PM   #5
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Originally Posted by lamarguy View Post
You're only partially correct. Some of the essential foam forming agents (e.g., polypeptides and iso-alpha-acids) are reabsorbed post-fermentation (i.e., not lost to krausen deposits) but the foam positive proteins do NOT get reabsorbed since they are, well, denatured. A subtle yet important distinction.
Until I see a plausible scientific explanation of this, I'm going to question it. How do foam positive proteins become denatured? What happens to their constituent proteins/amino acids? Why would some proteins get reabsorbed and others not? For that matter, why would some proteins denature and others not?
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Old 01-01-2010, 03:37 PM   #6
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For that matter, why would some proteins denature and others not?
Different proteins denature under different circumstances. This is readily observed in cooking. Egg whites, fish and red meat proteins denature and different temperatures when heat is the catalyst for denaturing. Fish protein can be denatured via low pH (ceviche) more readily than red meat protein.

Perhaps more important is that some proteins once denatured will never return to their native state, others will.

Now, I'm no chemist and I can't exhaustively explain what is going on in beer with regards to foam positive proteins, but I have to point out that you aren't exhaustively justifying your position any more than Jamil, me or Lamarguy are ours. Lamarguy's source authors are chemists so at least we have appeal to authority on our side.
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Old 01-01-2010, 05:01 PM   #7
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Until I see a plausible scientific explanation of this, I'm going to question it.
I'm not a chemist, so I can't help answer all of your questions but I can present supporting evidence that foam-positive proteins are denatured upon foam creation.

For example, take beating an egg white:

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All proteins, including those in egg white, are made of long chains of amino acids which are similar to beads on a string. In a raw egg, these chains are raveled up in a specifically arranged compact mass. Chemical bonds and interactions between the amino acids within each protein hold this mass in a specific shape and stop it from unraveling...

The physical stress of beating the egg white can create a foam. There are two types of physical stress caused by the beating of the egg whites with a whisk, the first being that the whisk drags the liquid through itself creating a force that unfolds the protein molecules. This process is called denaturation. The second stress comes from the mixing of air into the whites which causes the proteins to come out of their natural state. These denatured proteins gather together where the air and water meet and create multiple bonds with the other unraveled proteins and thus becomes a foam holding the incorporated air into place.
If you'd like a detailed explanation for exactly which proteins are denatured, this should be a good paper to read - Protein Denaturation in Foam: II. Surface Activity and Conformational Change.

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As part of a study of protein denaturation in foam we have measured the surface tension and the changes in protein structure occurring at the interface for lysozyme, pepsin, BSA, YADH, IgG, and catalase. The apparent CMC values were found to be dependent on the size and rigidity of the molecule. The variability of protein damage at a gas–liquid interface in foam was assessed using these proteins. The foams were produced under controlled conditions in a bubble column and were found to induce conformational changes in the protein molecules, but no fragmentation or disassociation of subunits occurred. Tertiary structural changes were detected in all the proteins studied, with some proteins forming aggregates. For pepsin, the secondary structure was also found to be altered. Enzyme solutions were used to determine the degree of biological activity retained after foaming for proteins with different structural characteristics. The more rigid proteins were found to display a low surface activity and a low degree of damage in foam. Pepsin suffered the highest rate of damage, which is thought to be a result of its inability to refold following denaturation.
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Old 01-02-2010, 01:11 PM   #8
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That's interesting, lamarguy. The 2nd article abstract shows that there is a mechanism for protein denaturation in foam at a liquid gas interface. It also shows that there is variation among proteins regarding how much, if at all, they denature in foam. Here are the proteins from the study:

lysozyme = antimicrobial enzyme- common in eggwhite
pepsin = proteolytic enzyme
BSA = bovine serum albumin
YADH = yeast alcohol dehydrogenase (enzyme - interconverts alcohols to aldehydes and ketones)
IgG = anti-body in blood
catalase = enzyme converting H2O2 to H2O and O2.

So none of these specifically relate to beer foam. I find this statement interesting:

Quote:
The more rigid proteins were found to display a low surface activity and a low degree of damage in foam.
I wonder how "rigid" are beer foam proteins?

It's good to finally see a potentially plausible explanation. I can't, however, say that I believe it in regards to beer foam until I see a specific study on it. It just doesn't add up. Why don't those who produce foam from aeration see a significant decrease in beer foam stability? Why did pjj2ba's experiment in the other thread have the results it did?
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Old 01-03-2010, 08:01 PM   #9
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I tend to think that these proteins get "used up" is mostly fake. I remember reading a scientific article where, in order to study the head-forming proteins, they bubbled gas into the beer, scraped off the foam, put it in water, bubble gas into that and so on until they had clean proteins. Why would that work if these proteins denature so easily?

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Old 01-04-2010, 01:41 AM   #10
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Denaturing is not always an end state!!!! More than just a few proteins can refold themselves after being denatured. It partly depends on what forces denatured the protein in the first place - heat, whipping, pH, salt concentration - and the other components of the solution/matrix. Also, many of the foam positive proteins, have been modified by the mashing process and fermentation so they behave differently from the native forms.

I don't know specifically about the proteins in beer, but I do know about proteins in plants. Some of these can definitely re-nature after being denatured. I recall that one of the foam positive protein is a HSP (heat shock protein). These come in a variety of sizes. These proteins still are not well understood, but one function that has been shown is to help to re-nature other proteins, typically in response to stresses

I think that as long as the foam proteins make it into the fermentor, then you are good to go. If you have a lot of foaming beforehand and don't let it reabsorb and leave it behind, then you are robbing yourself of potential foam. I've got some extra wort right now, maybe I'll half fill some clear glass bottles, autoclave and then SEAL them and redo the shake experiment. In my previous experiment, the flasks were just covered with foil, so maybe the air exposure had an influence

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